Redox flow batteries are suitable for grid scale storage applications due to their capabilities of scaling power and capacity independently, and charging and discharging for thousands of cycles with reduced performance losses as compared with conventional battery technologies. Redox flow battery cells generally employ two different types of membrane separators: ion exchange membranes (IEMs) and microporous membranes. IEMs generally exhibit higher ion selectivity but higher resistivity and higher cost, while microporous membranes are less expensive and exhibit lower resistivities but are associated with poor ion selectivity. Operating with separators having low ion selectivities can lower overall efficiency of a redox flow battery.
Some redox flow batteries employ a hybrid separator, such as VANADion™-20 membranes, including both an IEM and a microporous membrane layer laminated side by side in order to attempt to exploit the higher ion selectivity of the IEM layer, while mitigating overall resistivity and cost with the microporous membrane layer.
The inventors herein have discovered various issues with the above systems. Namely, due to the hydrophobicity of the microporous membrane layer, the microporous membrane can be difficult to thoroughly wet with the aqueous electrolyte, and can become infiltrated with air and other gas bubbles diffusing therein. The presence of gas bubbles in the microporous membrane layer can cause substantial increases in resistivity of the redox flow cell battery, which lowers battery efficiency. Even with thorough wetting, redox flow battery cells including microporous membrane layers can often exhibit increased resistivity, especially during charging mode operation.
The issues described above may at least partially be addressed by a method of operating a redox flow battery, including maintaining a positive electrode compartment pressure greater than a negative electrode compartment pressure, and maintaining a cross-over pressure less than a membrane break-through pressure, wherein the cross-over pressure equals the negative electrode compartment pressure subtracted from the positive electrode compartment pressure.
In this way, ionic resistance across the separator can be maintained at a lower level by reducing gas bubbles trapped therein while reducing separator break-through, thereby increasing performance of the redox flow battery system.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.